microsat wrote:In testing our experimental motors for ignition/re-ignition in space we built an 200mm vacuum chamber and simply use an aluminum plate with a silicon gasket. When the vacuum is applied, it holds the cap on yet when we do ignition, the cap blows away harmlessly.

Yes, this is the simplest way to do it, but I prefer the “chamber” (pipe) to be mounted horizontally with fall-away covers on both ends. Then, even when the engine fails catastrophically during a test (and you know it will now and then), the pipe can not become pressurized by the explosion because both ends open instantly. While the pipe might be shattered by an engine failure, it will not itself explode like a pipe bomb. 200 mm, however, sounds like a tight fit. The bigger the pipe (sorry, “chamber”), the better. Your safety is in your hands.

If you are planning a high-altitude balloon launch, a cryogenic vacuum test is the most authentic for simulating the long slow climb to launch altitude. Just chill the engine with dry ice (frozen carbon dioxide) well before the test.

Jay wrote: 200 mm, however, sounds like a tight fit. The bigger the pipe (sorry, “chamber”), the better. Your safety is in your hands.

Our short 100mm test motor we evaluate for space ignition easily fits into the 200mm vacuum chamber we built and is in the ground during testing. While we've never had an 'explosion' with this small test motor we still test it in the ground. A failure will simply blow things into the ground and vertically in open air. Somehow, a motor exploding and shattering a horizontally mounted pipe around it does not sound that safe but simply adds more fragments flying in all directions.

Jay wrote: If you are planning a high-altitude balloon launch, a cryogenic vacuum test is the most authentic for simulating the long slow climb to launch altitude. Just chill the engine with dry ice (frozen carbon dioxide) well before the test.

We have used both CO2 and liquid nitrogen in some of our experimenting and while CO2 would be adequate simulating a HAB launch it is not cryogenic. It would be of value to know what temperatures this stage would be experiencing after a 6 minute flight to 100,000 meters prior to its ignition.

microsat wrote:Somehow, a motor exploding and shattering a horizontally mounted pipe around it does not sound that safe but simply adds more fragments flying in all directions.

Sorry to be unclear, “shattered” was a poor choice of words. I was only speaking hypothetically, but the possibility of the pipe being fractured or cracked is what I meant to convey. Nothing goes flying, except out the ends of course! The pipe I’m talking about is very large and very heavy and definitely very safe.

microsat wrote:We have used both CO2 and liquid nitrogen in some of our experimenting and while CO2 would be adequate simulating a HAB launch it is not cryogenic. It would be of value to know what temperatures this stage would be experiencing after a 6 minute flight to 100,000 meters prior to its ignition.

The atmosphere between 80,000 and 90,000 meters, what little there is of it, actually has the coldest naturally occurring temperatures on Earth at roughly 175K (~ -100C or -145F). The vanishingly thin air at 100,000 meters is practically balmy at about 200K (-75C or -100 F). That’s actually slightly warmer than frozen carbon dioxide.

If you’re contemplating a balloon launch from 30,000 meters (100,000 feet) after a leisurely ascent, the temperature would be a cozy 225K (-48C or -55F). If you’re talking about the ignition of a second stage at 100,000 meters (~ 328,000 feet) after a six-minute rocket ride from the comparatively sweltering surface, the brief flight time and supersonic aerodynamic heating of the outer skin would render the outside air temperature practically irrelevant.

Considering this range of temperatures, liquid nitrogen, at 77K (-196C or -321F) is definitely much too cold to simulate a high-altitude balloon launch or the high-altitude ignition of a second stage. Now’s the time for “shattering” to come to mind!

I was not aware “cryogenic” had crystallized into such a hard definition. Thanks for the update.

Jay wrote:I was not aware “cryogenic” had crystallized into such a hard definition. Thanks for the update.

Jay

In my workings with liquid CO2, oxygen, nitrogen and hydrogen, CO2 and dry ice were never considered to be 'cryogenic'.Most people I know use liquid oxygen and nitrogen as the 'warmest' commonly used things considered to be cryogenic.http://en.wikipedia.org/wiki/Cryogenics

Definitions evolve. A long time ago in a previous century a computer was a person hired to do computations and cryogenic simply meant to make something colder than could be found in the environment, but then again my first tablet was made of clay and the only font was cuneiform! Thanks again for the update.